The downlink of the 3GPP Long Term Evolution (LTE) cellular wireless communication system is based on Orthogonal Frequency Division Multiplex (OFDM) transmissions, which uses time and frequency resource units for transmission. The smallest time-frequency resource unit, called Resource Element (RE), consists of a single complex sinusoid frequency (sub-carrier) in an OFDM symbol. For the purpose of scheduling transmissions to the different User Equipments (UEs), the resource elements are grouped into larger units called Physical Resource Blocks (PRBs). A PRB occupies a half of a subframe, called “slot”, consisting of six or seven consecutive OFDM symbol intervals in time domain (0.5 millisecond in total), and twelve consecutive sub-carrier frequencies in frequency domain (180 kHz in total).
Each PRB is indicated by a unique index nPRBϵ[0, NRBDL−1] denoting the position of the sub-band that the PRB occupies within a given bandwidth, where NRBDL−1 is the total number of PRBs within the bandwidth. The maximum number of PRBs NRBmax,DL, associated with the largest LTE bandwidth (20 MHz), is 110. The relation between the PRB number nPRB in the frequency domain and resource elements (k,l) in a slot is nPRB=└k/NscRB┘.
The LTE Rel-8/10 defines a Physical Downlink Control Channel (PDCCH) as a signal containing information needed to receive and demodulate the information transmitted from a serving cell, called eNodeB in LTE terminology, to a UE through the Physical Downlink Shared Channel (PDSCH). The PDCCH is transmitted in a control region that can occupy up to four OFDM symbols at the beginning of each subframe, whereas the remaining of the subframe forms the data region used for the transmission of the PDSCH channel.
The LTE Rel-11 supports a new control channel scheduled within the time-frequency resources of the downlink data region. Unlike the legacy PDCCH, this new feature, known as Enhanced Physical Downlink Control Channel (EPDCCH), has the distinct characteristic of using Demodulation Reference Signals (DMRS) for demodulation and, consequently, the ability to associate each EPDCCH with a specific mobile station, called UE in LTE terminology, i.e. DMRS signals are UE-specific.
The EPDCCH transmission can be either localized or distributed with the granularity of one PRB pair. With localized transmission, the EPDCCH for a UE is preferably transmitted over a single PRB pair (or, in some cases, over a few consecutive PRB pairs) scheduled by the associated eNodeB based on channel quality indicator (CQI) feedback information (i.e., by means of frequency selective scheduling); with distributed transmission on the other hand, the EPDCCH is transmitted over multiple PRB pairs spread over the downlink system bandwidth to achieve frequency diversity. The latter scheme is useful if there is no feedback from the mobile station or the available feedback is not reliable, although more resources (i.e. PRBs) are locked for EPDCCH transmission.
The EPDCCH design is based on a UE specifically configured search space. In particular, for a given mobile station, the serving cell (e.g. eNodeB in LTE) can allocate up to K=2 sets of physical resource units, called EPDCCH sets in LTE terminology, each consisting of a group of M={2, 4 or 8} PRB pairs, where M is not necessarily the same when two EPDCCH sets are allocated. Each EPDCCH set can be configured for either localized or distributed EPDCCH transmission. The unit block for EPDCCH multiplexing and blind decoding is the Enhanced Control Channel Element (ECCE), which consists of a block of resource elements in a PRB pair. When EPDCCH is transmitted, a plurality of ECCEs can be aggregated together based on the payload size and coding rate of the transmitted EPDCCH creating different aggregation levels, e.g. one, two, four, eight and sixteen. Therefore, one PRB pair can contain one or more ECCEs depending on the ECCE size and the mapping rule used to map EPDCCH to the PRB pair. For EPDCCH demodulation, four DMRS antenna ports 107-110 can be used. In order to reduce the detection complexity, the antenna port used for EPDCCH transmission shall be known to the mobile station. One way to indicate the used antenna ports to the mobile station is an implicit association between antenna ports and useful ECCEs. Several other methods for antenna port associations have been discussed, and the latest agreement is that with localized allocation, each ECCE index is associated by specification with one antenna port.
A key feature yet to be finalized is a method for signalling, to the mobile station, the location within the system frequency bandwidth for downlink transmissions of the sets of physical resources, i.e. PRB pairs, configured for the enhanced downlink control channel transmission. The information to be signalled consists of the indices of individual physical resource block (PRB) pairs grouped into one or two sets. While the signalling will be performed in higher layer of the system such as in the Radio Resource Control (RRC) layer, the detailed resource allocation method has not been decide. So far, prior art for allocating physical resources for the enhanced downlink control channel in the related art 3GPP LTE system has focused on two main approaches.
A first straightforward prior art method is to signal one bitmap for each allocated EPDCCH set. The bitmap associated to an EPDCCH set consists of one bit for each physical resource block (PRB) pair in the system bandwidth, where the bit is set to a specified value (e.g., 1) if the corresponding PRB pair is part of the EPDCCH set associated to the bitmap. This method offers the maximum flexibility in terms of EPDCCH set allocation as it allows to addressing any PRB pair in the downlink system bandwidth for each allocated set. The drawback is a significant overhead when the system bandwidth is large. For instance, in the related art LTE system, the largest system bandwidth consists of 110 PRB pairs (20 MHz), therefore resulting into up to 220 bits to be signalled to each mobile station.
An alternative prior art method was proposed to reduce the signalling overhead (Huawei, HiSilicon “EPDCCH resource allocation”, R1-124162, San Diego, USA, Oct. 8-12, 2012). This method is inspired to the data resource allocation type 1 of the related art LTE system, as it considers groups of resource blocks (i.e. RBGs in LTE terminology), and individual PRB pairs within an RBG are indicated with a bitmap. The inventive step is the definition of an EPDCCH resource subset consisting of individual PRB pairs in RBGs spread over the system bandwidth as the basic resource allocation unit for an EPDCCH set, i.e., an EPDCCH set consists of one or more EPDCCH subsets. Both RGB size and EPDCCH subset size depend on the system bandwidth as in Table 1.
TABLE 1Dimensioning of EPDCCH subset and RBG groups.Number ofSystemSubsetsubset perNumber of Overhead perbandwidth sizeRBG groups/RBGEPDCCH (RB)(RB)RBG sizegroups set (bits)6213415223525226850434710044610
An EPDCCH set is then signalled by indicating the EPDCCH subsets forming the EPDCCH set, where each EPDCCH subset is signalled using two bitmaps: a first bitmap indicating the allocated EPDCCH RBG group(s); and a second bitmap indicating the EPDCCH subset(s) in the allocated EPDCCH group(s), where the bitmap is common for all the indicated EPDCCH RBG group(s). The method has the merit to significantly reduce the signalling overhead required to signal an EPDCCH set compared to the case of full bitmap. For instance, Table 1 shows that each EPDCCH set can be signalled with 10 bits in a 100 PRB bandwidth. However, the bitmap structure used to reduce the signalling overhead introduces severe limitations to the flexibility of the resource allocation both in terms of which PRB pairs can be selected for an EPDCCH set (the second bitmap must be common to all EPDCCH RBG groups indicated in the first bitmap) and in terms of the minimum size of the EPDCCH set for large system bandwidth. For instance, with a 50 or 100 PRB pair bandwidth the smallest EPDCCH set supported consists of four PRB pairs instead of two, which cannot fulfil the current EPDCCH design requirements in the related art LTE system. Similar methods to reduce the signalling overhead by means of bitmaps addressing groups of PRB pairs, at the expense of the resource allocation flexibility, have been proposed by others, such as (NEC Group, “ePDCCH PRB configuration”, R1-124293, San Diego, USA, Oct. 8-12, 2012), (CATT, “EPDCCH set configuration”, R1-124102, San Diego, USA, Oct. 8-12, 2012.), (Samsung, “Design Aspects for EPDCCH Sets”, R1-124376, San Diego, Oct. 8-12, 2012) and (LG Electronics, “Details of EPDCCH set configuration”, R1-124322, San Diego, USA, Oct. 8-12, 2012).
Hence, there is a need in the art for an improved method that meets the flexibility of a full bitmap solution for each allocated set of physical resources for downlink control channel signals transmission while reducing the signalling overhead.